The present invention relates to a VLF test generator for generating a high voltage with a low frequency for testing the insulation of capacitive loads, in particular power cables, having an oscillator part which generates at an output a high voltage which has a high frequency and is modulated with a low frequency, and a demodulator connected thereto for demodulating the high voltage and recovering the low frequency therefrom.
Testing with high voltages of a very low frequency (VLF) in the tenth of a hertz range is now established practice for power testing of the insulation of highly capacitive loads such as buried cable systems. Unlike mains frequency or higher frequency test voltages, VLF test voltages bring about only slight reactive power in the capacitive load, such that the test generator may be made correspondingly smaller; and, in comparison with previously used direct voltage tests, VLF test voltages prevent any build-up of harmful space charges and residual charges in the cable system, which on subsequent operation could result in dielectric breakdowns.
Generating suitable VLF test voltages in the high voltage range, i.e. of up to several hundred kilovolts, is however not in any way straightforward, as high voltage transformers are not feasible for such low frequencies. The most varied circuits have thus already been proposed for VLF test generators, but all of them either involve highly complex circuitry or have costly or fault-prone components.
DE 103 33 241 B, for example, discloses a VLF test generator of the initially stated type which uses a variable transformer with a motor-driven adjustment means in order to amplitude-modulate a mains frequency high voltage by periodic adjustment of the transformer. The amplitude-modulated high voltage is stepped up and then, with the assistance of a demodulator, the modulation frequency is recovered as a VLF high voltage. The demodulator is formed by a diode rectifier whose conducting-state direction can be changed over and which changes over at each half-wave of the low frequency in order to reverse the capacitive load with each half-wave. A switchable discharging resistor is connected in parallel to the capacitive load in order to assist the reversal. Such a load-parallel discharging resistor does however result in elevated power loss and/or requires additional switching electronics with correspondingly increased costs, weight and cooling requirements.
The object of the invention is to overcome the disadvantages of the known prior art and to provide a VLF test generator for the generation of low-frequency high voltages which can be produced simply and inexpensively, has a low weight for mobile, on-site use, has a low power loss and accordingly a low cooling capacity.